depend on new gnureadline instead of readline on OS X

this is the same package under a different name,
to avoid the stdlib-overriding behavior that relied on easy_install's unpleasantness.
remove no-longer-necessary dirty hack for readline on OS X

just import gnureadline

(yay!)
adjust dependency check for gnureadline on OS X

- always depend when making a bdist (for consistency)
- otherwise only depend if missing (e.g. pip install -e)
only depend on readline package in CPython

shouldn't try to install it on PyPy

not sure about PyReadline on PyPy on Windows
use rlineimpl in terminal.console
Merge pull request #5181 from minrk/gnureadline

use gnureadline on OS X